1. Field of the Invention
This invention relates to the recovery of one or more aromatic hydrocarbon compounds (aromatic or aromatics) from a mixture of such aromatics and one or more non-aromatic hydrocarbon compounds (non-aromatic or non-aromatics) using solvent extraction, extractive distillation, and reboiled stripping techniques.
2. Description of the Prior Art
Although, for the sake of clarity and brevity, this invention will sometimes be described hereinafter in respect of the recovery of benzene alone, it is to be understood that this invention is applicable as well to the recovery of aromatic products in general such as toluene, xylenes (ortho, meta, and/or para isomers), ethyl benzene, and the like, individually or in any combination or mixture of two or more thereof.
Heretofore, the recovery of one or more aromatic products from a feed composed of a mixture of at least one aromatic and at least one non-aromatic has been practiced by employing a solvent extraction process on that feed followed by extractive distillation and stripping steps to separately recover the solvent and an aromatic extract (concentrate). The extract was then subjected to one or more distillation steps to separate there from one or more individual aromatic products as ultimate (final) products of the overall process.
In simplest terms, in a solvent extraction process, a feed material containing, for example, benzene mixed with at least one non-aromatic is mixed with a solvent that (i) has a boiling temperature (boiling point) that is quite different from the boiling point of benzene, (ii) preferentially absorbs benzene from the feed, and (iii) naturally physically separates from the undissolved feed. The solvent preferentially absorbs the benzene from the feed. The benzene rich solvent is then subjected to extractive distillation and stripping steps to separate the solvent from the benzene and produce a benzene extract (aromatic extract) from which is recovered a benzene product that is purified of non-aromatics in the feed down to a predetermined maximum non-aromatic weight content (specification). Most, but not all, non-aromatics are physically separated from the aromatics in forming the extract. The remainder of the feed which contains the vast majority, but not all, of the non-aromatics is separately recovered as a raffinate stream for other processing.
The benzene rich aromatic extract contains minor amounts of non-aromatics. The non-aromatic content of the extract often needs to be reduced in order to meet the purity specification set for the benzene product, and all other aromatic products, desired to be produced from this extract. The maximum amount (weight percent, parts per million, etc.) of non-aromatic impurities allowed to be present in a given benzene product is the “predetermined maximum specification” (predetermined specification, maximum specification, or specification). This predetermined specification can vary from process to process and product to product depending on the quality (non-aromatic purification level) desired for a particular benzene product. The purer the aromatic product desired for a given use of that product, the smaller the total non-aromatic content of that product.
Also heretofore, in the practice of the foregoing solvent extraction process, in an attempt to maintain the non-aromatic content of the aromatic products below their predetermined specifications, the aromatic extract was analyzed by conventional gas chromatography to determine the total non-aromatic content of that extract, and process operating changes were made in order to try to maintain the non-aromatic content of the aromatic product below its specification. These operating changes include changes (i) in the feed rate to the solvent extraction step, (ii) the weight ratio of feed to solvent and (iii) the weight ratio of backwash (defined hereinafter) to feed and other process parameters.
However, practice has shown that aromatic solvent exchange processes are notorious for their difficulty in predicting that a desired final aromatic product will meet (be below) its predetermined maximum specification. For example, it is common that the same change in backwash rate to the solvent extractor will not affect the extraction process in the same way every time thereby making it highly unpredictable as to whether such a change will allow the process to meet the specifications for its aromatic products.
As a result of such unpredictability and the rigid specifications, these processes have been heretofore operated in a manner such that their benzene and/or other individual aromatic products have a non-aromatic content that is very far below their predetermined maximum specifications, i.e., lower in non-aromatic content (purer) than necessary to meet the predetermined specifications. That is to say, because of the foregoing unpredictability, the prior art practice has been to make aromatic products that in some cases were unnecessarily pure for the chosen use for those products.
Manufacturing aromatic products that have a non-aromatic content that is far below that which is needed for a desired use of the product reduces the producing capacity of the process and uses more energy to produce a unit of product than would otherwise be used if the process was operated in a manner that produced all its aromatic products closer to their predetermined maximum specifications, i.e., closer to optimal. For example, if a benzene product has a non-aromatic content that is half, 50 weight percent below, its predetermined maximum specification, and it is purer than is necessary for the desired use for that product. This production of a benzene product that is purer than necessary reduces the operating capacity of the process as measured by the amount of feed per unit time that can be introduced to the process, and utilizes more energy in doing so because less product is produced for the same energy expenditure. Thus, producing one or more aromatic products each having substantially less of a non-aromatic content than is required is not optimal operation for the process.
This invention addresses the problem of unpredictability in the operation of such aromatic solvent extraction processes to meet predetermined specifications, and, in so doing, provides a solvent extraction process that not only has greater operational predictability, but also operates more closely to its optimal capacity utilization at minimum energy use.